A typical patch element of a patch antenna is in the form of a flat rectangular or circular metal microstrip or patch on the surface of a dielectric, with the dielectric on a conducting ground plane. The patch element acts as a parallel plate, microstrip transmission line serving as an antenna by giving in-phase linearly or circularly polarized radiation. The patch element is fed, for example, by a coaxial feed. A coaxial feed comprises, a conducting central conductor encircled concentrically, first, by a dielectric, and then, by an outer conductor serving as a conducting shield. The ground plane of the typical patch element is connected to the shield. In the past, a known method of feeding the patch element required the center conductor of the coaxial feed to connect at a natural feed point on the patch element.
The natural feed point on a patch element is located closer to one edge of the patch element. A typical null point is on a polar axis of symmetry for the patch element.
A stacked antenna, is of compact, low profile construction, with stacked patch elements operating at separate frequency bands. A patch element that is directly fed by a coaxial feed has its ground plane connected to a portion of the coaxial feed that is referenced to ground. The stacked patch element lacks inherent isolation of its operating band of frequencies due to the use of a common feed. Accordingly, the patch elements of a stacked patch antenna are poorly isolated, which increases the complexities of tuning and frequency band separation by adding circuit components.
In the past, it was unknown to couple null points of stacked patch elements with a coaxial feed, since excitations fed at the null point tend to reform, before being radiated, rendering the patch element ineffective as a normal mode antenna.